This invention relates to floating gate transistors and methods of forming the same. This invention also relates to methods of enhancing data retention of floating gate transistors.
Floating gate transistors are utilized in some semiconductor a memory cells. One type of memory cell that uses a floating gate transistor is a flash erasable and programmable read only memory (EPROM). A floating gate transistor typically includes a tunnel dielectric layer, a floating gate, an interlayer dielectric and a control gate or word line. Source/drain regions are formed operatively adjacent the floating gate and within semiconductive substrate material. A floating gate transistor can be placed in a programmed state by storing charge on the floating gate of the floating gate transistor. Typically, a large, e.g. 25 volts, between the control gate and the substrate allows some electrons to cross the interlayer dielectric and charge the floating gate. The xe2x80x9cdata-retentionxe2x80x9d of a floating gate transistor refers to the ability of the transistor to retain its charge over a period of time. Charge can be lost, undesirably, through electron migration from the floating gate through various adjacent materials. One problem which has confronted the industry is electron migration through the interlayer dielectric material immediately above the floating gate. The thickness of the interlayer dielectric material has an impact on the ability of a floating gate to retain its charge. Thinner regions of the interlayer dielectric material provide undesired migration paths for electrons to leave the programmed floating gate relative to other thicker regions of the interlayer dielectric material. Hence, non-uniformity in the thickness of the interlayer dielectric material is undesirable.
A contributing factor to a non-uniformly thick interlayer dielectric material is the presence of a large number of grain boundaries at the interlayer dielectric/floating gate interface. Conductive doping of the floating gate, as is desirable, undesirably increases the number of interface grain boundaries, which in turn, increases the chances of having a non-uniformly thick interlayer dielectric.
This invention grew out of concerns associated with improving the data retention characteristics of floating gate transistors.
Floating gate transistors and methods of forming the same are described. In one implementation, a floating gate is formed over a substrate. The floating gate has an inner first portion and an outer second portion. Conductivity enhancing impurity is provided in the inner first portion to a greater concentration than conductivity enhancing impurity in the outer second portion. In another implementation, the floating gate is formed from a first layer of conductively doped semiconductive material and a second layer of substantially undoped semiconductive material. In another implementation, the floating gate is formed from a first material having a first average grain size and a second material having a second average grain size which is larger than the first average grain size.